1. Field of the Invention
This invention relates to a printer which is provided with a plurality of electrophotographic photosensitive mediums and which forms images of plural colors.
2. Description of the Prior Art
A stationary ghost light flux is created when a photosensitive medium is scanned by a light beam modulated by a recorded image information signal. The reason that such stationary ghost light flux is created is described in U.S. Pat. No. 4,040,737. That is, part of the light beam having entered the photosensitive medium is reflected by the photosensitive medium. This reflected light travels back through a focusing lens and again enters the rotatable polygonal mirror, and is again reflected by the polygonal mirror and enters the focusing lens. Thus, the light flux having left the focusing lens is stationary in spite of the polygonal mirror being rotated. Such stationary light flux is called a stationary ghost light flux. If the stationary ghost light flux overlaps the scanning locus of the light beam on the photosensitive medium, there will be formed a noise in the image.
To prevent the above-noted incovenience, the technique disclosed in U.S. Pat. No. 4,040,737 is adapted in the conventional apparatus shown in FIG. 1 of the accompanying drawings.
In FIG. 1, a semiconductor laser 1 emits a laser beam L modulated correspondingly to an image information signal. This laser beam L is scanned by a polygonal mirror 3 rotatively driven at a constant speed by a motor 2. The laser beam L deflected by the polygonal mirror 3 is focused to a drum-like electrophotographic photosensitive medium 6 rotated in the direction of arrow, by a lens 4 having an f-.theta. characteristic. Designated by 5 is a mirror for deflecting the optical path. Since the laser beam L is scanned by the polygonal mirror 3, it moves on the photosensitive medium 6.
To separate the stationary ghost light flux from the optical path of the laser beam which is to scan the photosensitive medium 6, the laser beam L is caused to enter the polygonal mirror 3 from a direction inclined by an angle -.alpha. with respect to an imaginary plane perpendicular to the rotary shaft 10 of the polygonal mirror 3. In other words, the polygonal mirror 3 is rotated about the shaft 10 inclined by an angle other than 90.degree. with respect to the laser beam L entering the polygonal mirror 3. Thus, the stationary ghost light flux is separated from the optical path of the laser beam scanning the photosensitive medium and is intercepted relative to the photosensitive medium by a light-intercepting member 8. The laser beam scanning the photosensitive medium 6 passes through a slit-like opening 8' provided in the light-intercepting member 8.
Reference numeral 1' designates the line of intersection between an imaginary plane perpendicular to the rotary shaft 10 of the polygonal mirror 3 and an imaginary plane containing the axis of the laser beam L entering the polygonal mirror 3 and parallel to the shaft 10. The fact that the laser beam L is inclined by an angle -.alpha. with respect to the imaginary plane perpendicular to the shaft 10 means that the angle formed between the laser beam L and the imaginary line of intersection 1' is -.alpha..
In any case, if the laser beam L is inclined by the angle -.alpha. with respect to the imaginary line of intersection 1', the movement locus A of the laser beam L on the photosensitive medium 6 (the scanning line on the photosensitive medium) will be curved as shown.
Assuming that as shown in FIG. 2 of the accompanying drawings, the maximum amount of curvature of this curved scanning line is .DELTA.x, the focal length of the lens 4 is f and the full scanning width on the photosensitive medium 6 is l, the maximum amount of curvature .DELTA.x is ##EQU1## Assuming that as an example, -.alpha.=-40', f=220 mm and l=250 mm, then .DELTA.x=0.14 mm. If -.alpha.=40', the direction of curvature of the scanning line will be opposite to the direction of curvature of the scanning line A as indicated by broken line (A) in FIG. 2.
Heretofore, the above-described degree of curvature has been difficult to sense by the naked eye and has offered no problem in a monochromatic printer. However, in the case of a color laser printer, if the degrees and directions of curvature of the scanning lines formed on the photosensitive drums for respective colors differ from each other, there will occur color misregistration on a transfer medium. The misregistration between images of various colors is very conspicuous and the quality of the color image is reduced. The allowance of the amount of color misregistration (the amount of misregistration between a first color image and a second color image) is usually 0.1 mm. Accordingly, if the direction of curvature of the first scanning line on a first photosensitive medium and the direction of curvature of the second scanning line on a second photosensitive medium differ from each other or the degrees of curvature of the first scanning line and the second scanning line greatly differ from each other and the amount of color misregistration exceeds 0.1 mm, the quality of the image will be deteriorated.
In FIG. 1, the photosensitive medium 6 is charged by a charger 11 and thereafter is exposed to the laser beam L. Thereby, an electrostatic latent image is formed on the photosensitive medium 6. This latent image is developed by a developing device 12 which supplies toner to the photosensitive medium. The visible image thus obtained is transferred to a transfer medium 7 by a transfer charger 13.